No one seems to have mentioned this VOIP Mobile outfit now owned by Microsoft.
It offers an application for Android, BlackBerry OS, iOS, and Windows Phone that allows users to make free phone calls and send free text messages to anyone who has the application installed. Viber works on both 3G and WiFi networks
http://www.viber.com/You will need a mobile data plan to use it if not connected by wifi.
Now has over 100,Million Users.
http://news.cnet.com/8301-1035_3-57510236-94/viber-hits-100-million-users-partners-with-nokia-on-wp7/